Faraday rotator and optical attenuator

ABSTRACT

A Faraday rotator and an optical attenuator using the Faraday rotator in which both a fixed magnetic field parallel to and a valuable magnetic field perpendicular to the optical axis are applied to Faraday elements, said optical axis being in the &lt;111&gt; direction of single crystal of garnet, characterized in that three single crystals of garnet of substantially the same thickness having the Faraday effect are used to form Faraday elements and the Faraday elements are arranged in such a manner that a variable magnetic field is applied to one of the Faraday elements, over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (−1−12) plane on the outermost circumference or a plane equivalent thereto in the chart, whereas a variable magnetic field is applied to the remaining two elements, over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (−101) plane on the outermost circumference or a plane equivalent thereto in the chart. Temperature dependence of optical decay thus is improved. Also, positioning means for Faraday rotator improves the polarization dependence loss.

INDUSTRIAL FIELD OF THE INVENTION

This invention relates to a device for adjusting the angle of Faradayrotation (Faraday rotator) and also to an optical attenuator using sucha device. The Faraday rotation angle-adjusting device and opticalattenuator according to the present invention are specifically used inoptical transmission communication systems. The invention, inparticular, improves the temperature dependence of Faraday rotationangle and narrows the scatter of variations among products in the angleof Faraday rotation with externally applied variable magnetic fields.Moreover, the invention reduces the variable magnetic field required toachieve a desired low-current characteristic or specific amount ofattenuation (proportional to the angle of Faraday rotation).

PRIOR ART

Owing to the striking expansion of transmission capacities, there is agrowing demand for high-density-wavelength multiplex transmissionsystems. This has accordingly increased the need for variable opticalattenuators that dynamically adjust the quantity of light required forthe systems. Applications for the attenuators include the control oflight quantities for individual channels and simultaneous attenuation ofmultiplex light rays. Among those optical attenuators there is one typethat utilizes magneto-optical effect. That type usually involves alayout in which a component capable of changing the angle of Faradayrotation a light beam is disposed between a polarizer and an analyzer.With one such component for changing the Faraday rotation angle,external magnetic fields are applied from two or more differentdirections to a single crystal of garnet having a Faraday effect so asto make the composite external field variable, whereby the Faradayrotation angle of the light that passes through the single crystal ofgarnet is controlled (Registered Japanese Patent 2,815,509).

To be more specific, the Registered Japanese Patent 2,815,509 disclosesan optical attenuator which, while keeping a fixed magnetic fieldgreater than the saturation magnetic field of a single crystal of garnetapplied to the crystal in a direction parallel to the optical axis bymeans of a permanent magnet, applies a variable magnetic field to thecrystal in a direction perpendicular to the optical axis by anelectromagnet, thereby changes the composite magnetic field vector, andchanges the direction of magnetization and hence the Faraday rotationangle of the single garnet crystal so that the quantity of light coupledto the fiber on the leaving side can be controlled.

Another method is known as a means of decreasing the temperaturedependence of optical attenuators, which comprises applying externalmagnetic fields in the directions where the amount of change of theFaraday rotation angle due to the temperature dependence of the anglebetween the direction of magnetization of Faraday elements and thedirection of the beam and the amount of change of the Faraday rotationangle due to the temperature dependence with the Faraday effect aredifferent in code from each other and the absolute value of eitheramount is less than twice that of the other amount, whereby the changesof Faraday rotation angle with temperature are restricted. (JapanesePatent Application Kokai No. 11-249095).

PROBLEMS THAT THE INVENTION IS TO SOLVE

With the foregoing in view, a plurality of optical attenuators wereexperimentally fabricated. The experiments presented a problem of widescatter among the specimens of the temperature dependence values of theattenuation and the electromagnet field perpendicular to the opticalaxis required to attain the maximum attenuation. Another problem thatarose was the requirement of a large variable magnetic field (and hencea large driving current) to achieve a desired attenuation (proportionalto the angle of Faraday rotation).

Therefore, the solution of these problems in accordance with the presentinvention is to provide an optical attenuator that narrows the scatterof characteristics among devices, reduces the temperature dependence,and possesses good low-current characteristics.

Also, in view of the foregoing, we experimentally fabricated a pluralityof optical attenuators and found that they show, in common, that anincrease in the quantity of attenuation is accompanied with an increasein the polarization dependence loss (hereinafter abbreviated to “PDL”)up to more than one decibel at peak. The value is by far the greaterthan those of non-polarization-dependent optical isolators, the opticaldevices that similarly take the advantage of magneto-optical effect andare already in wide use with optical transmission systems. Typical PDLvalues of the non-polarization-dependent optical isolators are of theorder of 0.1 dB.

A study of the difference revealed that, when birefringent elements areused as a polarizer and an analyzer, ordinary and extraordinary rayspass through a Faraday element along different paths and accordingly thedistributions of magnetic fields that are applied to different portionsof the Faraday element vary too. Factors that can subtly influence themagnetic field distributions are, for example, the direction ofapplication of variable magnetic field, shape and size of the yoke of anelectromagnet that produces the variable field, and the relativepositions of the yoke and Faraday element.

FIG. 22 illustrates, by way of example, a conventional member forattaching a Faraday element to an optical attenuator. The member has acutout 20 for receiving a yoke and an opening 15 formed in the centerfor the passage of a light ray. A Faraday element recess 21 is formed atthe bottom of the cutout 20 in alignment with the opening 15, and aFaraday element (not shown) consisting of one or more garnet crystalplates is received in the recess. The Faraday element is secured inplace with a thermosetting or ultraviolet-curing resin filled inresin-filling ports 16, 17 open to the cutout 20. Yokes 10, 10 ofelectromagnets are inserted on both sides of the opening 15 into thecutout 20 and are similarly fixed with a hardening resin. There is nomeans of positioning the inner ends of the yokes 10 of electromagnets,and the yokes simply secured with respect to the Faraday element fail tomaintain a constant positional relationship. Moreover, the combined areaof the yokes relative to the Faraday element is restricted. These andother factors are deemed responsible for the problem of increased PDL.

Therefore, another problem that the present invention is to solve aresettled by the provision of an optical attenuator having favorable PDLcharacteristics.

MEANS OF SOLVING THE PROBLEMS

The present invention provides a Faraday rotator in which magnetic fieldis applied to Faraday elements, optical axis of said Faraday elementsbeing in the <111> direction of single crystal of garnet, characterizedin that

three single crystals of garnet of substantially the same thicknesshaving a Faraday effect are used to form the Faraday elements and theFaraday elements are arranged in such a manner that a first magneticfield is applied to one of the Faraday elements, in the directionperpendicular to a plane in a range extending 5 deg. each to the leftand right of the line connecting the (111) plane in the center of astereographic projection chart with the (−1−12) plane on the outermostcircumference or a plane equivalent thereto in the chart, whereas asecond magnetic field is applied to the remaining two elements, in thedirection perpendicular to plane in a range extending 5 deg. each to theleft and right of the line connecting the (111) plane in the center ofthe stereographic projection chart with the (−101) plane on theoutermost circumference or a plane equivalent thereto in the chart. Eachof said first and second magnetic fields may be a composite magneticfield formed by a pair of magnetic fields.

The present invention also provides a Faraday rotator in which both amagnetic field parallel to and a magnetic field perpendicular to theoptical axis are applied to Faraday elements, said optical axis being inthe <111> direction of single crystal of garnet, characterized in that

three single crystals of garnet of substantially the same thicknesshaving a Faraday effect are used to form the Faraday elements and theFaraday elements are arranged in such a manner that a first magneticfield is applied to one of the Faraday elements, in the directionperpendicular to a plane in a range extending 5 deg. each to the leftand right of the line connecting the (111) plane in the center of thestereographic projection chart with the (−1−12) plane on the outermostcircumference or a plane equivalent thereto in the chart, whereas asecond magnetic field is applied to the remaining two elements, in thedirection perpendicular to a plane in a range extending 5 deg. each tothe left and right of the line connecting the (111) plane in the centerof the stereographic projection chart with the (−101) plane on theoutermost circumference or a plane equivalent thereto in the chart.

The present invention also provides an optical attenuator including apolarizer and an analyzer disposed, respectively, before and after aplurality of Faraday elements, in which a variable magnetic field isapplied to Faraday elements, in such manner that the variable magneticfield can change the angle of Faraday rotation of a light beam andcontrol the quantity of light transmitted, said optical axis being inthe <111> direction of single crystal of garnet, characterized in thatthree single crystals of garnet of substantially the same thicknesshaving a Faraday effect are used to form Faraday elements and theFaraday elements are arranged in such a manner that a variable magneticfield is applied to one of the Faraday elements, in the directionperpendicular to planes over a range extending 5 deg. each to the leftand right of the line connecting the (111) plane in the center of thestereographic projection chart with the (−1−12) plane on the outermostcircumference or a plane equivalent thereto in the chart, whereas avariable magnetic field is applied to the remaining two elements, in thedirection perpendicular to planes over a range extending 5 deg. each tothe left and right of the line connecting the (111) plane in the centerof the stereographic projection chart with the (−101) plane on theoutermost circumference or a plane equivalent thereto in the chart.

The variable magnetic field may be a composite magnetic field appliedfrom a pair of magnetic fields at least one of which is variable.

The present invention also provides an optical attenuator including apolarizer and an analyzer disposed, respectively, before and after aplurality of Faraday elements, in which both a magnetic field parallelto and a magnetic field perpendicular to the optical axis are applied tothe Faraday elements, one of the magnetic fields being fixed and theother being variable, so that the composite magnetic field thereof canchange the angle of Faraday rotation of a light beam and control thequantity of light transmitted, said optical axis being in the <111>direction of single crystal of garnet, characterized in that

three single crystals of garnet of substantially the same thicknesshaving a Faraday effect are used to form Faraday elements and theFaraday elements are arranged in such a manner that a variable magneticfield is applied to one of the Faraday elements, in the directionperpendicular to planes over a range extending 5 deg. each to the leftand right of the line connecting the (111) plane in the center of thestereographic projection chart with the (−1−12) plane on the outermostcircumference or a plane equivalent thereto in the chart, whereas avariable magnetic field is applied to the remaining two elements, in thedirection perpendicular to planes over a range extending 5 deg. each tothe left and right of the line connecting the (111) plane in the centerof the stereographic projection chart with the (−101) plane on theoutermost circumference or a plane equivalent thereto in the chart.

In the Faraday rotator and optical attenuator defined above, preferablythe magnetic field parallel to the optical axis is a fixed magneticfield generated by permanent magnets and the magnetic fieldperpendicular to the optical axis is a variable magnetic field generatedby electromagnets.

According to the invention, the Faraday rotator is composed of acombination of three Faraday elements of specific orientation, and notonly the temperature dependence of Faraday rotation angle is reduced butalso the scatter of Faraday rotation characteristics is controlled.

To be more specific, the Faraday rotator and attenuator according to thepresent invention are limited in temperature variation of theattenuation (that depends on the Faraday rotation angle) and, moreover,the scatter of the attenuation-temperature variation among the specimensis narrow. Furthermore, the scatter of the peak current value(corresponding to the variable magnetic field required to achieve themaximum change in the Faraday rotation angle) among the specimens issmall and, in addition, low-current characteristics can be attained(that is, the driving current to generate a variable magnetic fieldnecessary for obtaining a certain amount of attenuation or Faradayrotation angle may be small).

Another Means of Solving the Problems

The present invention also provides an optical attenuator which controlsthe angle of Faraday rotation of a light beam that passes through asingle crystal of garnet having the Faraday effect by applying twoexternal magnetic fields, fixed and variable, from two differentdirections, characterized in that at least one garnet crystal having theFaraday effect is used as a Faraday element, and a member for holdingthe Faraday element in place has a stopper to position the front ends ofyokes of electromagnets that apply the variable magnetic field to andaround the holder, with respect to the direction of field application.

The invention also provides an optical attenuator as defined abovecharacterized in that the member for holding the Faraday element inplace has a pair of positioning grooves to position the front ends ofyokes of electromagnets that apply the variable magnetic field to andaround the holder, with respect to the direction of light beam.

The invention further provides an optical attenuator according to claim1 characterized in that the yokes of the electromagnets that apply thevariable magnetic field have a front end plane each perpendicular to thedirection of the variable field with a cross sectional area no less than1.7 times that of the plane of the Faraday element perpendicular to thedirection of the variable field.

To be more concrete, the invention provides an optical attenuatorcomprising a member formed with a first groove extending across theoptical axis and also formed with an opening open to the first groovealong the optical axis, a Faraday element disposed in the first groovein alignment with the optical axis, said member having a pair of secondgrooves formed on both sides of, and close to, the Faraday element, saidsecond grooves extending across the first groove and in the directionnormal to the optical axis, and a pair of electromagnets that produces avariable magnetic field, said magnets having yokes the ends of which arefitted in the pair of second grooves on both sides of the Faradayelement, the bottoms of the second grooves serving as a stopper forpositioning the front ends of the yokes.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a stereographic projection chart of crystal faces centered onthe (111) plane.

FIG. 2 is a graphic representation of the relation between theelectromagnet field and Faraday rotation angle in the route ofmagnetization rotation.

FIG. 3 is a perspective view showing the construction of an opticalattenuator.

FIG. 4 is a schematic view of a 1-mm cubic single crystal of garnet asbeveled.

FIG. 5 gives graphs showing the characteristics of optical attenuatorspecimens, i.e., their values of temperature dependence onattenuation-current characteristics values, when a variable magneticfield was applied to one of the three single crystals of garnet in eachspecimen, over the line connecting the (111) plane in the center withthe (−1−12) plane on the outermost circumference of the stereographicprojection chart, whereas a variable magnetic field was applied to theremaining two elements, over the line connecting the (111) plane in thecenter with the (−101) plane on the outermost circumference of thechart.

FIG. 6 is a table giving the characteristics of optical attenuatorspecimens, i.e., their peak current values and maximum values ofattenuation variation over the temperature range of 0-65° C., when avariable magnetic field was applied to one of the three single crystalsof garnet in each specimen, over the line connecting the (111) plane inthe center with the (−1−12) plane on the outermost circumference of thestereographic projection chart, whereas a variable magnetic field wasapplied to the remaining two elements, over the line connecting the(111) plane in the center with the (−101) plane on the outermostcircumference of the chart.

FIG. 7 gives graphs showing the characteristics of optical attenuatorspecimens, i.e., their values of temperature dependence onattenuation-current characteristics values, when a variable magneticfield was applied to all three Faraday elements of each specimen, on theline connecting the (111) plane in the center with a plane inclined atan angle of 26° from the (−1−12) plane on the outermost circumferencetoward the (−101) plane of the stereographic projection chart.

FIG. 8 is a table giving the characteristics of optical attenuatorspecimens, i.e., their peak current values and maximum values ofattenuation variation over the temperature range of 0-65° C., when avariable magnetic field was applied to all three Faraday elements ofeach specimen, on the line connecting the (111) plane in the center witha plane inclined at an angle of 26° from the (−1−12) plane on theoutermost circumference toward the (−101) plane of the stereographicprojection chart.

FIG. 9 gives graphs showing the characteristics of optical attenuatorspecimens, i.e., their values of temperature dependence onattenuation-current characteristics values, when a variable magneticfield was applied to one of the three single crystals of garnet in eachspecimen, over the line connecting the (111) plane in the center withthe (−1−12) plane on the outermost circumference of the stereographicprojection chart, whereas a variable magnetic field was applied to theremaining two elements, over the line connecting the (111) plane in thecenter with the (−211) plane on the outermost circumference of thechart.

FIG. 10 is a table giving the characteristics of optical attenuatorspecimens, i.e., their peak current values and maximum values ofattenuation variation over the temperature range of 0-65° C., when avariable magnetic field was applied to one of the three single crystalsof garnet in each specimen, over the line connecting the (111) plane inthe center with the (−1−12) plane on the outermost circumference of thestereographic projection chart, whereas a variable magnetic field wasapplied to the remaining two elements, over the line connecting the(111) plane in the center with the (−211) plane on the outermostcircumference of the chart.

FIG. 11 shows the temperature dependence characteristics of a total of444 optical attenuators fabricated from 16 lots of garnet crystals inaccordance with the procedures described in the examples of theinvention.

FIG. 12 is a schematic view showing the construction of an opticalattenuator.

FIG. 13 gives the results of computation of the relation between PDL andthe discrepancy between Faraday rotation angles of split light beams andPDL.

FIG. 14 shows the field application zone of electromagnets that apply avariable magnetic field.

FIG. 15 gives the results of computation of magnetic field distributionin a field application zone.

FIG. 16 illustrates samples of a symmetrical pair of electromagnet yokesand an asymmetrical pair of electromagnet yokes.

FIG. 17 gives the results of evaluation of the characteristics ofdistribution within the incidence area of PDL and attenuation insymmetrical and asymmetrical electromagnet yoke samples.

FIG. 18 depicts an element holder having a yoke stopper andyoke-positioning grooves; (a) being a left side view, (b) a front view,(c) a back view, (d) a plan view, and (e) a bottom view.

FIG. 19 presents schematic views showing how an element is joined to anelement holder and how a pair of yokes are joined under pressure; (a)being a left side view, (b) a back view, and (c) a perspective view asseen from below the rear side.

FIG. 20 is a view illustrating single crystal of garnet one millimetersquare in size after a corner beveling.

FIG. 21 gives characteristics, i.e., PDL and attenuation currentcharacteristics and PDL values at the attenuation of 18.5 dB, of samplesembodying the present invention.

FIG. 22 illustrates an element holder not provided with a yoke stopper;(a) being a right side view, (b) a back view, (c) a plan view, and (d) abottom view.

FIG. 23 gives characteristics, i.e., PDL and attenuation currentcharacteristics and PDL values at the attenuation of 18.5 dB, of samplesin comparative examples.

PREFERRED EMBODIMENTS OF THE INVENTION

In the following, preferred embodiments are described. It is noted thatthe embodiments are described in connection with a composite magneticfield composed of two magnetic fields, one being fixed and the otherbeing variable, it should be noted that so long as the desired vector ofvariable magnetic field may be generated a single electromagnet or aplurality of electromagnets, or a combination of permanents andelectromagnets may be adopted. In the following, it should be understoodthat although the direction of the composite magnetic field is notspecified the direction is in perpendicular to the plane designated bythe crystallographic planes. That is, a statement that a variablemagnetic field is applied in over a line connecting the (111) plane to(k,l,m) (k, l, m are specific integers) means that the magnetic field isapplied in the direction between <111> (inclusive) and <k,l,m>(inclusive).

In accordance with the present invention, Faraday elements are arrangedin such a way that a variable magnetic field of electromagnets isapplied to one of three elements, over the line connecting the (111)plane in the center with the (−1−12) plane on the outermostcircumference of the stereographic projection chart, whereas a variablemagnetic field is applied to the remaining two elements, over the lineconnecting the (111) plane in the center with the (−101) plane on theoutermost circumference of the chart. FIG. 1 is a stereographicprojection chart centered on the (111) plane of a single crystal ofgarnet. Any given plane of the garnet crystal may be represented by adot in this stereographic projection chart. Owing to the symmetry of thecrystal structure, a plane equivalent to the (−1−12) plane emerges atevery 120 deg. on the outermost circumference, and a plane equivalent tothe (−101) plane comes at every 60 deg. Here the plane equivalent to the(−1−12) plane is either the (−12−1) or (2−1−1) plane and the planeequivalent to the (−101) plane is any of the (−110), (01−1), (10−1),(1−10), and (0−11) planes. Negative indices of the crystal planes areindicated by indices each with a minus symbol.

FIG. 2 graphically represents the results of measurement of Faradayrotation angle and magnetic field in the directions of electromagnetfield application with different garnet crystal orientations. The graphreveals that the Faraday rotation angle varies widely with theelectromagnet field depending on the direction of magnetic fieldapplication. Thus the reproducibility of attenuation characteristicswith the applied variable magnetic field can be enhanced by distinctlyspecifying the relation between the orientation of garnet single crystaland the direction of electromagnet field applicable to the garnetcrystal. It has now been found possible to reduce with goodreproducibility the temperature dependence of the attenuationcharacteristics with the applied variable magnetic field, when, inconformity with the invention, a variable magnetic field is applied toone of the three single crystals of garnet in each specimen, over theline connecting the (111) plane in the center with the (−1−12) plane onthe outermost circumference of the stereographic projection chart,whereas a variable magnetic field is applied to the remaining twoelements, over the line connecting the (111) plane in the center withthe (−101) plane on the outermost circumference of the chart.

As will be clear from a comparison between the Examples of the inventionand Comparative Examples to be given below, the narrowing of scatter ofthe temperature dependence of attenuation according to the invention ispresumably attributable to the greater tolerance than in the prior artof the angular deviation of different directions of magnetic fields thatare applied to specific orientations.

FIG. 3 illustrates the basic construction of a Faraday rotatorcomprising a combination of three Faraday elements and two differentmagnets and of an optical attenuator using the rotator. The arrangementis such that a light beam travels, from the beam incidence side forward,through a polarizer 2, a set of three Faraday elements 3, 4, 5 asdefined above, an analyzer 6, and a phase compensation prism 8, in theorder of mention. To the Faraday elements 3, 4, 5 is applied a fixedsaturation magnetic field in the optical axis by permanent magnets 7, 7.Also, a variable magnetic field is applicable in the direction normal tothe optical axis by electromagnets 10, 10. These three Faraday elementsand two kinds of magnets are joined to constitute a Faraday rotator.

EXAMPLE 1

An optical attenuator of the basic construction shown in FIG. 3 wasmade, and the relation between the direction of application ofelectromagnet field and the orientation of garnet crystal, the relationbetween the amount of attenuation and electromagnet field, and thetemperature dependency were examined. The arrangement was such that abeam of light could pass through a polarizer, a plurality of Faradayelements, an analyzer, and a phase compensation prism, located in theorder of mention. The Faraday elements were arranged so that the lightbeam was incident perpendicularly to the (111) planes of the elements. Amagnetic field parallel to the light beam was applied by two permanentmagnets to the Faraday elements, while electromagnets applied a magneticfield perpendicular to the beam. As the Faraday rotator was kept in thestate of magnetic saturation, the current being supplied to theelectromagnets was varied so as to change continuously the angle ofFaraday rotation of the beam being transmitted and accordingly changethe quantity of light of the leaving beam. The relative angle of thepolarization planes of the polarizer and analyzer through which the beamwas to pass was set to 105 deg.

The Faraday element was fabricated in the following way. A singlecrystal of garnet was grown on a nonmagnetic garnet substrate by theliquid phase epitaxial technique. With reference to the orientation flatsurface formed on the nonmagnetic garnet substrate, the resultingcrystal was slitted at intervals of 11 mm in parallel with andperpendicularly to the <−1−12> direction, and the upper right corner ofthe side of each slitted piece normal to the <−1−12> direction wasbeveled. Next, the substrate was removed by grinding, and thesemifinished pieces were heat treated at 1030° C. in air for 20 hours.The heat treatment was done for the purpose of reducing the growthinduced magnetic anisotropy. The pieces were then mirror polished to afinish thickness at which the angle of Faraday rotation was about 32deg. Following this, nonreflective films were formed on both sides ofthe pieces. Next, the 11 mm-square garnet single crystal pieces formedwith the nonreflective films were cut into chips 1 mm square in thedirections parallel to and perpendicular to the four sides of eachpiece. The upper right of the side perpendicular to the <−1−12>direction of each chip was beveled (FIG. 4). The beveling was intendedto make the crystal orientation of each chip after the scissiondistinct. Three such 1 mm-square chips of garnet single crystal wereused as Faraday elements.

Two optical attenuators each of three different garnet single crystallots were made. In each attenuator Faraday elements were arranged insuch a way that a variable magnetic-field was applicable to one of thethree elements, over the line connecting the (111) plane in the centerwith the (−1−12) plane on the outermost circumference of thestereographic projection chart, whereas a variable magnetic field wasapplicable to the remaining two elements, over the line connecting the(111) plane in the center with the (−101) plane on the outermostcircumference of the chart.

The temperature dependence values (at 0°, 25°, and 65° C.) of theattenuation-current characteristics of these test specimens weremeasured. The results are shown in FIG. 5. The attenuation-currentcharacteristics and temperature dependence tendencies were quitefavorably reproduced by the individual specimens. The graphs alsoindicate the attenuation-temperature variations as computed from themeasured values of attenuation.

With each specimen the peak current at which the peak of attenuation at25° C. was attained and the maximum value of the variation inattenuation over the range of 0-65° C. at attenuation below 20 dB weredetermined. The results are summarized in FIG. 6. With the individualsamples the attenuation-current characteristics and temperaturedependence values were reproduced with very good results.

Comparative Example 1

As Comparative Example 1, optical attenuators were made with a Faradayelement arrangement such that electromagnet fields were applicable indifferent orientations. The Faraday elements were arranged so that allthree were superposed in the same orientation and an electromagnet fieldwas applicable to the line connecting the (111) plane in the center witha plane inclined at an angle of 26° from the (−1−12) plane on theoutermost circumference toward the (−101) plane of the stereographicprojection chart. Three optical attenuators were made each from twodifferent garnet single crystal lots, and the temperature dependencevalues (at 0°, 25°, and 65° C.) of the attenuation-currentcharacteristics of these test specimens were measured. The results areshown in FIG. 7.

With each specimen the peak current at which the peak of attenuation at25° C. was attained and the maximum value of the variation inattenuation over the range of 0-65° C. at attenuation below 20 dB weredetermined. The results are summarized in FIG. 8. The graphs indicatethat, with the individual samples, the peak current and temperaturedependence varies widely from specimen to specimen.

It is obvious from the foregoing that in the Example of the presentinvention not only the attenuation-temperature variation is limited butalso the scatter of the attenuation-temperature variation among thespecimens tested was small. In addition, the scatter of peak currentvalues (corresponding to the variable magnetic fields required to obtainthe maximum change in the angle of Faraday rotation) is restricted and,moreover, low-current characteristics are obtained (which means that theamount of a driving current to produce a variable magnetic fieldrequired to obtain a specific attenuation amount or angle of Faradayrotation can be kept small).

Comparative Example 2

As Comparative Example 2, a plurality of optical attenuators of variedgarnet single crystal lots were made. In each attenuator Faradayelements were arranged in such a way that a variable magnetic field wasapplicable to one of the three elements, over the line connecting the(111) plane in the center with the (−1−12) plane on the outermostcircumference of the stereographic projection chart, whereas a variablemagnetic field was applicable to the remaining two elements, over theline connecting the (111) plane in the center with the (−211) plane onthe outermost circumference of the chart. The temperature dependencevalues (at 0°, 25°, and 65° C.) of the attenuation-currentcharacteristics of these test specimens were measured. The results areshown in FIG. 9. With each specimen the peak current at which the peakof attenuation at 25° C. was attained and the maximum value of thevariation in attenuation over the range of 0-65° C. at attenuation below20 dB were determined. The results are summarized in FIG. 10. Althoughthe individual samples gave favorable values of temperature dependenceupon attenuation, the peak-current value at which the maximumattenuation was achieved was about 70 mA, as much as about 1.8 timesgreater than the values of the specimens fabricated in accordance withthe present invention, suggesting the effectiveness of the presentinvention in lowering the current requirement.

EXAMPLE 2

Following the procedure described in Example 1 of the present invention,a total of 444 optical attenuators were made from 16 lots of garnetcrystals. Their temperature dependence values were determined, theresults being summarized in FIG. 11. Despite the possibility of scatterof approximately ±5 deg. in crystal orientation as the scatter offabrication including the tolerances in the cutting direction and insecuring the elements in place, the figure indicates narrow scatter oftemperature dependence and favorable reproducibility.

As has been described above in connection with FIGS. 1-11, the presentinvention improves the reproducibility of attenuation characteristics ofoptical attenuators with respect to applied variable magnetic fields,reduces the temperature dependence of the attenuation characteristicswith respect to applied variable fields, and enhances thereproducibility. Further, the invention makes it possible to achievelow-current characteristics.

The present invention is also concerned with an optical attenuator whichcontrols the quantity of light that transmits through a single crystalof garnet having a Faraday effect by applying two external magneticfields, one fixed and the other variable, from opposite directions tothe crystal and thereby making the Faraday rotation angle of the ray oflight that transmits through the crystal variable. By way of example, abasic construction of an optical attenuator is shown in FIG. 12. Thearrangement is such that a beam of light passes through a polarizer 2, aplurality of Faraday elements 1 each consisting of a garnet crystal, ananalyzer 6, and a phase compensation prism 8, located in the order ofmention, so that an attenuated beam of light emerges as indicated by anarrow. External magnetic field application means comprises a pair ofpermanent magnets 7, 7 disposed on opposite sides of the Faradayelements 1 and which jointly apply a magnetic field parallel to the axisof light and a pair of electromagnets 10, 10 (only the front ends oftheir yokes being shown) which apply a variable magnetic fieldperpendicular to the light axis. In order to attain independence frompolarization, wedge-shaped polarization separation elements are used forthe polarizer 2 and analyzer 6. Those elements are made of birefringentcrystals. As a consequence, the incoming light beam is separated intoordinary and extraordinary rays in the polarizer 2 located on theincidence side of the Faraday elements (garnet crystals), and then inthe polarized state the separate rays enter the garnet crystals. As therays travel through the garnet crystals, their directions ofpolarization are rotated by the Faraday effect. This behavior is taughtin the Registered U.S. Pat. No. 2,815,509. In brief, as they travelthrough the garnet crystals, the ordinary and extraordinary raysseparated as a result of polarization pass different paths across thecrystals. Thus, theoretically, different Faraday rotation angles of thetwo rays separated by polarization cause a PDL.

In FIG. 13 are graphically shown the results of calculation of therelation between PDL and the discrepancy between separated rays with anattenuation value of 18.5 dB. The relation is represented by amathematical expression or Formula 1;

where φ is the relative angle of the optical axis of the wedge-shapedpolarization separation elements, Δθ is the discrepancy between theFaraday rotation angles of ordinary and extraordinary rays, Att is theattenuation value, and θfAtt is the Faraday rotation angle at which adesired attenuation value is attained:

PDL=|−10 log└cos²{φ−(θ_(fAtt)+Δθ)}┘−Att|  [Formula 1]

The formula indicates that the PDL increases as the discrepancy betweenthe Faraday rotation angles expands.

Ordinarily the angle of Faraday rotation varies with the externalmagnetic field that is applied to the single crystals of garnet. Toreduce the discrepancy between the angles of Faraday rotation of therays separated by polarization, therefore, it is necessary to apply asuniform a magnetic field to the crystals as possible. In reality,however, the strength of the variable magnetic field that is applied tothe electromagnets is difficult to control, because the strength dependson various factors such as the size and shape of the yokes of theelectromagnets and their relative position to the garnet crystals.

A magnetic field application zone of electromagnets that apply avariable magnetic field (i.e., the zone where Faraday elements aredisposed) is shown in FIG. 14. A variable current is passed throughcoils 11 of conductor wire wound on both yokes 10 to produce a variablemagnetic field between the yokes. The variable magnetic field is appliedto one or more Faraday elements of garnet crystal interposed in theregion between the yokes so as to adjust the attenuation value of light.

The angular distribution of the vector of the external magnetic fieldapplied to this region was computed. The results are shown in FIG. 15.As conditions for the computation, the distance between the yokes 10, 10of the two electromagnets was set to 1 mm, the cross sectional area ofthe front end of each yoke to 1×1 mm, the number of turns of wire ineach coil to 1,500 turns, the diameter of the wire to 0.1 mm, and theelectromagnet current to 70 mA. A model arrangement was made in whichtwo permanent magnets 7, 7 were located at the front and rear of theyokes to apply a magnetic field parallel to the optical axis (see FIG.12). The dimensions of the permanent magnets were 3.5 mm in outsidediameter, 1.3 mm in inside diameter, and 1.0 mm long. The distancebetween the center of each yoke and the center of each permanent magnetwas 3.5 mm. It will be appreciated that the most favorably distributedregion is at and around the centers. For this reason it is presumedthat, for the reduction of the PDL characteristic, the element should belocated as precisely in the center between the electromagnet yokes aspossible.

The above was experimentally confirmed. Experiments were made to see ifthe PDL characteristic is improved in a uniform magnetic field. Twosample models of optical attenuators, one having symmetricalelectromagnetic yokes and the other asymmetrical electromagnetic yokes,were made. Each sample was evaluated in respect of the distributions ofPDL and attenuation in the incidental plane. The symmetricalelectromagnet yoke sample used left and right yokes arrangedsymmetrically and the asymmetrical electromagnet yoke sample used oneyoke located about 100 μm more distant from the other yoke.

The evaluation results are graphically represented in FIG. 17. To sumup, the closer to the center between the yokes the less the attenuationwas. With the asymmetrical yoke sample, the minimum peak was about 100μm off the center. The results were in agreement with the results ofmagnetic field analysis in which the closer to the center between theyokes the smaller the absolute value of the magnetic field. The PDLdistribution too became less as it approaches the center between theelectromagnets. As with the attenuation, the minimum peak was also about100 μm off the center. This indicates that in the center betweenelectromagnets where the magnetic field distribution is favorable, PDLtoo is improved.

From the foregoing it is clear that a favorable distribution of theexternally applied magnetic field in the Faraday elements is essentialfor the improvement in the PDL characteristic. For the improved fielddistribution it is also effective to locate the element as precisely inthe center between the electromagnet yokes as possible and allow thefront ends of the magnet yokes to have a cross sectional area greaterthan that of the Faraday element. In addition, the provision of a memberfor holding the Faraday element in position and of a stopper formed inthe periphery of the member for the insertion of yokes improves thesymmetry of the electromagnet yokes and enhance the PDL characteristicvalue. Further, the provision of the element-holding member formed withgrooves in the periphery of the member to position the electromagnetyokes with respect to the optical axis makes it possible for the Faradayelement to be disposed in the center between the electromagnet withrespect to the optical axis direction. With the yokes of theelectromagnets that apply the variable magnetic field, the larger thecross sectional area of the plane of each yoke end perpendicular to thedirection of the variable field compared with the cross sectional areaof the Faraday element perpendicular to the variable field, the betterthe uniformity of the magnetic field to which the Faraday element issubjected.

Moreover, the stopper formed in the element holder keeps the yokes outof contact with the element, enhancing the reliability against thermalexpansion due to temperature changes and against changes with the lapseof time.

An optical attenuator of the basic construction illustrated in FIG. 12was fabricated. The construction was such that a light beam indicated byan arrow could pass through a polarizer 2, a Faraday element 1consisting of three plates of garnet crystal, an analyzer 6, and a phasecompensation prism 8 to yield an attenuated light beam. The Faradayelement 1 was located in such a way that the light beam could beincident perpendicularly to the (111) plane of the element. To thisFaraday element was applied a magnetic field parallel to the light beamby means of two permanent magnets 7, 7 and a magnetic field normal tothe light beam was applied by electromagnets 10,10. While the Faradayelement 1 was being kept in a magnetically saturated state by means ofthe permanent magnets 7, 7, the current being supplied to theelectromagnets 10, 10 was varied, whereby the angle of Faraday rotationof the transmitted light could be continuously varied and the quantityof the emerging light beam be changed. The relative angle of thepolarization plane of the light passing through the polarizer 2 and theanalyzer 6 each was 105 deg.

The Faraday element was joined securely to an element-securing holder 12as shown in FIG. 18. To be more particular, the element holder 12 has afirst groove 20 which extends across the optical axis, a Faradayelement-holding stage 18 formed in the middle portion of the groove 20,an opening 15 formed in the Faraday element-holding stage 18 in thegroove 20 and along the optical axis, and a pair of yoke-positioninggrooves 14 formed close to the both sides of the Faraday element-holdingstage 18 and extending normal to the optical axis across the firstgroove. With respect to the Faraday element-holding stage 18, there aretwo resin injection holes 16, 17 formed in the walls of the first groove20, in alignment with each other. In the yoke-positioning grooves 14,the portion adjacent to the first groove 20 (the portion indicated bytwo-dot chain lines in FIG. 18(a)) constitutes a stopper portion 13 forthe yokes. Unlike the counterpart of the prior art illustrated in FIG.22, the first groove 20 serves as a groove for positioning the Faradayelement, where the yokes are held in position by utilizing theyoke-positioning grooves 14 and the stopper portion 13. Steps formedbetween the first groove 20 and one side faces of the yoke-positioninggrooves 14 as shown not only increases the area of the yoke stopperportion 13 but also locate the Faraday element in the centers of theyoked in the direction of the optical axis, thus making it possible toapply a uniform magnetic field to the Faraday element.

As shown in FIG. 19, the Faraday element 1 is fitted in the Faradayelement-holding stage 13 in the first groove 19, locat□ing the elementin alignment with the optical axis, and a curable resin is injected intothe stage through the resin injection holes 16, 17 to secure the elementin place. Next, the yokes 10 of the two electromagnets are fitted in thepair of yoke-positioning grooves 14 so as to sandwich the Faradayelement in between. The front ends of the yokes are pressed against theyoke-positioning stopper portion at the bottom of the grooves, and thecurable resin is injected into the grooves 14 to fix the yoke endssecurely. In this manner the yoke ends are precisely positioned. Also,the both side walls of the yoke-positioning grooves 14 allow the yokes10 to be positioned in the direction of the optical axis.

Further, as FIG. 18(a) and FIG. 19(b) indicate, the cross sectional areaof the stopper portion 13 of the yoke-positioning grooves 14 is largerthan that of the first groove 20, the distribution of the magnetic fieldapplied to the Faraday element 1 is made all the more uniform.

As described above, the element holder according to the presentinvention is utilized in securing electromagnet yokes in place, wherebythe symmetry of electromagnet yokes is enhanced. Moreover, because theyokes are kept out of contact with the element, the qualitativereliability of the assembly is improved. The electromagnet yokes usedfor the experiments had front end dimensions of 1.3 mm by 1.2 mm. Thecross sectional area of the electromagnet yokes was set to a value about1.7 times that of the element.

The Faraday element was fabricated in the following way. A singlecrystal of garnet was grown on a nonmagnetic garnet substrate by theliquid phase epitaxial technique. With reference to the orientation flatsurface formed on the nonmagnetic garnet substrate, the resultingcrystal was slitted at intervals of 11 mm in parallel with andperpendicularly to the <−1−12> direction, and the upper right corner ofthe side of each slitted piece normal to the <−1−12> direction wasbeveled. Next, the substrate was removed by grinding, and thesemifinished pieces were heat treated at 1030° C. in air for 20 hours.The heat treatment was done for the purpose of reducing the growthinduced magnetic anisotropy. The pieces were then mirror polished to afinish thickness (about 0.3 mm) at which the angle of Faraday rotationis about 32 deg. Following this, nonreflective films were formed on bothsides of the pieces. Next, the 11 mm-square garnet single crystal piecesformed with the nonreflective films were cut into chips 1 mm square inthe directions parallel to and perpendicular to the four sides of eachpiece. The upper right of the side perpendicular to the <−1−12>direction of each chip was beveled (FIG. 20). The beveling is intendedto clarify the crystal orientation of each chip after the scission.Three such 1 mm-square chips of garnet single crystal were used asFaraday elements.

The three Faraday elements were placed in the element holder having yokestopper, and were bonded securely in position with ultraviolet-curingresin injected through the upper and lower holes 0.7 mm in diametereach. The Faraday elements were fixed after positioning with care takennot to allow them to come out of place. In fixing the electromagnets,they were bonded in place with their yoke ends pressed against the yokestopper of the element holder. This enhanced the symmetry of the leftand right electromagnet yokes and ensured the stability of theyoke-to-yoke distance.

With the optical attenuators thus experimentally fabricated using theelement holder provided with a yoke stopper, their PDL and attenuationcurrent characteristics and the PDL values at the attenuation of 18.5 dBwere evaluated. The results are given in FIG. 21. As for the PDL currentcharacteristics, the maximum PDL value at the current level that gavethe attenuation peak was of the order of 0.5 dB. The PDL value at theattenuation of 18.5 dB, as the average of 14 sample attenuatorsfabricated, was 0.25 dB, a favorable characteristic value.

By way of comparison, optical attenuators were made using an elementholder not provided with a yoke stopper as shown in FIG. 22 andemploying electromagnets with yoke end dimensions of 1.0 mm by 1.2 mm.With these attenuators, the PDL and attenuation current characteristicsand PDL values at an attenuation of 18.5 dB were evaluated. FIG. 23summarizes the results. In respect of the PDL current characteristics,the maximum PDL value at the current level that yielded the attenuationpeak exceeded 1.2 dB. The average PDL value of 9 samples triallymanufactured was 0.53 dB at the attenuation of 18.5 dB. The individualPDL values were about twice the values of the samples made in accordancewith the present invention, and this demonstrates the effectiveness ofthe invention in improving the PDL characteristic.

As has been described above in connection with FIGS. 12-23, the presentinvention renders the variable magnetic field applicable to Faradayelements uniform and thereby improves the PDL characteristic.

[Description of symbols]  1, 3, 4, 5 Faraday elements  2 Polarizer  6Analyzer  7 Permanent magnet  8 Phase compensation prism 10Electromagnet 11 Coil of electromagnet 13 Yoke stopper 14Yoke-positioning groove 15 Opening 16, 17 Resin-filling ports 18Element-holding stage 20 Element holder

What is claimed is:
 1. An optical attenuator which controls the angle ofFaraday rotation of a light beam that passes through a single crystal ofgarnet having the Faraday effect by applying two external magneticfields, fixed and variable, from two different directions, characterizedin that at least one garnet crystal having a Faraday effect is used as aFaraday element, and a member for holding the Faraday element in placehas a stopper to position the front ends of yokes of electromagnets thatapply the variable magnetic field to and around the holder, with respectto the direction of field application, the member for holding theFaraday element in place having a pair of positioning grooves toposition the front ends of yokes of electromagnets that apply thevariable magnetic field to and around the holder, with respect to thedirection of light beam.
 2. An optical attenuator which controls theangle of Faraday rotation of a light beam that passes through a singlecrystal of garnet having the Faraday effect by applying two externalmagnetic fields, fixed and variable, from two different directions,characterized in that at least one garnet crystal having a Faradayeffect is used as a Faraday element, and a member for holding theFaraday element in place has a stopper to position the front ends ofyokes of electromagnets that apply the variable magnetic field to andaround the holder, with respect to the direction of field application,the yokes of the electromagnets that apply the variable magnetic fieldhaving a front end plane each perpendicular to the direction of thevariable field with a cross sectional area no less than 1.7 times thatof the plane of the Faraday element perpendicular to the direction ofthe variable field.
 3. An optical attenuator comprising a member formedwith a first groove extending across the optical axis and also formedwith an opening open to the first groove along the optical axis, aFaraday element disposed in the first groove in alignment with theoptical axis, said member having a pair of second grooves formed on bothsides of, and close to, the Faraday element, said second groovesextending across the first groove and in the direction normal to theoptical axis, and a pair of electromagnets that produce a variablemagnetic field, said magnets having yokes the ends of which are fittedin the pair of second grooves on both sides of the Faraday element, thebottoms of the second grooves serving as a stopper for positioning thefront ends of the yokes.